SaM - MEMORIZE (Ultrasonic Beams)

List of things to memorize:

SaM - Ultrasonic Beams

• Measurements & Measurement Types for Ultrasonic Beams
  • Transmission Mode
  • Echo Mode
    • Acustic Impedance
• Influence of the Source Geometry on the US Filed Shape-Type
• Example of Beams
• Focused Transducer
• Control of a Beam
• Beam Stearing
• Linear Scanning
• Real World Example of Ultrasonic Sensors
• CMUT Probes
• Absorption

Link to original

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SaM - Measurements & Measurement Types for Ultrasonic Beams

• $3$ Example of measurements:
  1. Non-Contact Measurement.
  2. Echography and Doppler Effect.
  3. Non Destructive Testing.
• $2$ Measurement Types:

1. Transmission.
   (2 devices that alternates between sending an reciving).
2. Echo mode. (the most important one)
   (1 device for both sensing and reciving).

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SaM - Transmission Mode for Ultrasonic Beams • C-Mode Image

• Transmission mode: using two synced transmitter(TX) and reciver (RX) we will obtain a C-mode image:
  
  
• You can reduce the frequency to see more in depth, or you can increase it to have a more clear image (but at surface level).
• TX and RX must operate at the same frequency.
• Sometimes we place the object in water, using water as a coupling medium, but this is not so easy to do.

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SaM - Echo Mode for Ultrasonic Beams

• In “echo mode” measurements, ==we have a single transducer== which acts as a transmitter (TX) and also as a receiver (RX).
• Example:
  
• Actuation phase:
  
  
• Modelling of the sensor part, ignoring the TX part:
  
• Time of flight of the returning echos:
  
• Time of flights:$\begin{array}{l}{t}_1=\frac{2l_1}{v_{l_1}}\\ t_2=\frac{2l_2}{v_{l_1}}+\frac{2(l_2-l_1)}{v_{l_2}}\end{array}$
• “Initial dead zone”: obstical closer than $\frac{T_P}{2}{v}_{l_1}$ cannot be seen.NOT_SURE_ABOUT_THIS (calculation made by myself)
• Surfaces too close to each other:
  
• TODO check tof and dead zones
• Time-of-flight-difference between the second and third material:$\Delta t=\frac{2\Delta l}{v_{l_2}}$
• Minimum distance:$\Delta l\gg \frac{N}{2}{\lambda }_w$
• General time of flight formula:$t_{TOF}=\frac{2l}{v}$
• Terminology:
  • $t$ : time.
  • $l$ : size, more specifiaclly length, of the matrial.
  • $v$ : propagation velocity of the wave.
  • ${\lambda }_w$ : wavelenght.
  • $N=\frac{T_P}{T}$
    • $T_P$ : duration of the pulse.
    • $T=\frac{1}{f_0}$ : period of the wave.
    • “The duration of the pulse $(T_p)$ is a multiple $(N)$ of the ++wave period** $(T)$”
      ⇒ $N\in \mathbb{N}$

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SaM - Acustic Impedance • Ultrasonic Lumped Parameter System

• For ultrasonic systems we have:
  • Flow quantity: ${\dot{u}}_1$
  • Effort quantity: $T_{11}$ (stress) for solids, $p$ (preassure) for liquids.
  • ==Acustic Impendance== $\left(\frac{\text{effort quantity}}{\text{flow quantity}}\right)$:$\begin{array}{lc}\text{fluids:}& Z=\frac{p}{{\dot{u}}_1}\\ \text{solids:}& Z=\frac{T_{11}}{{\dot{u}}_1}\end{array}$Or more generarly:$Z=\rho \cdot v_l$Where:
    • ${\dot{u}}_1$ represents “==how fast the wave changes form==”.
    • $v_l$ is the longitudinal propagation velocity.
    • Both are velocities, and as such are expressed in $\left[\frac{\text{m}}{\text{s}}\right]$.

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SaM - Influence of the Source Geometry on the Ultrasonic Filed Shape-Type

• Point source ⇒ spherical wave:
  
• ~Ex.: Transducer which is a Plane Piston Circular (more usual and real case):
  
  • Inside the cilinder (“Frensel Region” or “Near Field”): approximately planar.
  • Outside the cilinder (“Far Field Region”): behaving like a spherical wavefront.
• For the Transducer which is a Plane Piston Circular, if Ii find two obstacles within the “far region field”, both are seen by simultaneusly the probe and they appear as to be only one obstical, so they can’t be distinguished (with a still probe):
  

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SaM ~ Example of Ultrasonic Beams

(Skipped)

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SaM - Focused Transducer for Ultrasonic Beams

• Focused transducer:
  

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SaM - Control of an Ultrasonic Beam

• tranducer is formed by many different and independently excited elements.
  
• If we move the array of tranducers, as shown in the picture, we can emulate the foused beam structure and obtain the same effect.
• However, since the wave depends a linear relationship between time and space, I don’t need to physically move each transducer, because ==the only thing that is needed is to excite these single elements with a time shift==.

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SaM - Ultrasonic Beam Stearing

• Example:
  

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SaM - Linear Scanning for Ultrasonic Beams

• Example:
  

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SaM ~ Real World Example • Ultrasonic Sensors

(Skipped)

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SaM - Alternative to Piezoelectric Arrays for Ultrasonic Beams

• A possible solution, a possible alternative to piezoelectric arrays are the so-called CMUT probes.
  CMUT (Capacitive Ultrasonic Transducer)
• They use the MEMS technology, so they are less expensive, so you can obtain more complex devices.

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SaM - Absorption for Ultrasonic Beams

• In real materials you have “absorption”: the conversion of ultrasound energy to heat energy.
  This means that the amplitude of the wave decays when traveling in a media.